1. Field of the Invention
The present invention relates to data transmission of a mobile communication system. More particularly, the present invention relates to an apparatus and a method for scheduling a data rate at which a user equipment (UE) transmits data to a Node B.
2. Description of the Related Art
A mobile communication system is a general term for systems providing voice, data, or other types of information through a wireless network. The mobile communication systems can be classified according to multiplexing schemes used, an example of which is a Code Division Multiple Access (CDMA) mobile communication system which provides wireless mobile communication service using a CDMA scheme. For the CDMA mobile communication system, an IS-95 standard mainly for transmission/reception of voice signals was initially developed and an IMT-2000 standard for transmission of not only voice signals but also high speed data is now being discussed. Specifically, the IMT-2000 standard provides services such as high quality voice reproduction, dynamic image reproduction, Internet access service, etc.
In the CDMA mobile communication system, the same frequency band is used by multiple users, and multiplexing is implemented by spreading data by means of a specific code for each of the multiple users. An increase in the data transmission speed of a UE of each of the multiple users implies an increase in transmission power, and the transmission power increase may serve as an interference factor to other UEs. Therefore, it is necessary to discuss a method for reducing the interference factor by scheduling the data transmission speed of the UE.
In a network of the existing 2G or 3G mobile communication system, when a new UE accesses the network, a maximum data transmission speed allowable to the UE is determined in consideration of a reception signal level and noise rise of each UE and is reported to the newly accessed UE. Then, the UE sets the transmission speed in consideration of the reported maximum data transmission speed, the quantity of data to be transmitted, and priorities of the data. The UE transmits the data at the set transmission speed.
FIG. 1 illustrates a Transport Format Combination Set (TFCS) generated using interference levels of multiple Node Bs measured by a Radio Network Controller (RNC) and transmitted to each UE. The TFCS contains data transmission speed which is allowed for each UE receiving the TFCS. The TFCS includes Transport Format Combination (TFC) 0 through TFC10. TFC0 represents the highest data transmission speed and TFC10 represents the lowest data transmission speed. The UE selects a TFC in consideration of the received TFCS, buffer occupancy, and maximum transmit power Max_Tx_Pwr. FIG. 1 shows the UE's selection of TFC6 as indicated by arrow 102. As described above, the data transmission speed of the UE is determined through scheduling by the UE itself based on the received TFCS.
When the UE spontaneously determines the TFC, the RNC inevitably takes a long time to reflect a rise in noise of the Node B. The longer the time taken by the RNC in reflecting the rise in noise of the Node B which changes instantaneously, the more difficult it is to precisely reflect the rise in noise in the TFCS to be transmitted to a UE newly accessing a Node B controlled by the RNC.
Further, packet data having a burst data transmission characteristic has a larger noise rise dispersion than that of voice data, so the noise rise of the Node B shows an increased variance (i.e., dispersion).
FIG. 2 illustrates noise rise variance of the Node B according to time.
As shown in FIG. 2, the interference elements of the Node B are classified into thermal noise, interference of other Node Bs, interference by a voice channel, and interference on a packet channel. Variance in the thermal noise, the interference of other Node Bs, and the interference by a voice channel according to the passage of time are either predictable or very little. However, it should be noted that the interference on a packet channel has a large variance according to time. That is, it should be noted that most of the noise rise variance of the Node B is determined by the variance of the interference on a packet channel. In FIG. 2, ‘max’ signifies a maximum allowable interference level and ‘target_1’ signifies a target interference level reflecting variance of the interference level according to time. Also, ‘margin’ signifies the difference between the maximum allowable interference level and the target interference level. The ‘margin’ is determined according to the variance of the interference level. In other words, because the sum of the noise rise is not allowed to exceed the value ‘max’ in performing the scheduling, the ‘margin’ must be increased in proportion to the noise rise variance when the noise rise has a large variance. Therefore, an increase in the variance of the interference level causes an increase of the margin and a decrease in the variance of the interference level causes a decrease of the margin. However, a mobile communication system creates a large amount of interference with a packet channel, which causes a large variance in the interference level, thereby increasing the ‘margin’.
As described above, each UE determines by itself the data transmission speed, thereby increasing the noise rise variance and accordingly increasing the margin. This implies that the power which the Node B can assign for use of each UE for data transmission is reduced according to an increase of the ‘margin’. That is, as the ‘margin’ increases, inefficient use of radio resources increase. FIG. 2 also shows inefficient use of radio resources.
Hereinafter, the noise rise will be discussed. The noise rise can be expressed as shown in the following equation (1).
                    Noise_rise        =                                            I              or                        +                          I              oc                        +                          N              t                                            N            t                                              (        1        )            
In equation (1), Ior represents the power of reception signals transmitted from UEs located in a particular cell, Ioc represents the power of reception signals transmitted from UEs located in another cell, and Nt represents the power of noise.
FIG. 3 is a block diagram illustrating a general form of uplink data transmission from a UE to a Node B. The system shown in FIG. 3 includes an RNC 300, a Node B 302, and a UE 304.
The UE 304 requests a data rate and control information to the Node B 302 through an Enhanced Dedicated Physical Control Channel (E-DPCCH) as shown by arrow 306. The Node B 302 transmits the control information at a determined data rate in response to the request of the UE 304 as shown by arrow 310. Specifically, the control information and the data rate are determined by the RNC 300 and are then transmitted to the Node B 302 and then to the UE 304. The UE 304 transmits data by means of the received data rate and the control information as shown by arrow 308. Herein, the data is transmitted through an Enhanced Dedicated Physical Data Channel (E-DPDCH).
FIG. 4 illustrates a process in which a UE selects a TFC.
In step 402, the UE determines whether a UE pointer j has been received from the Node B or not. When the UE pointer j has been received, step 424 is executed. When the UE pointer j has not been received, step 404 is executed. Hereinafter, a Node B pointer and the UE pointer j will be described with reference to FIG. 5. The Node B pointer 502 refers to a TFCS assigned and transmitted to a particular Node B belonging to a cell controlled by an RNC. The UE pointer j 504 refers to a TFC which the Node B assigns to the UE in consideration of the TFCS transferred from the RNC and a received interference level, etc. Usually, the UE transmits data by means of the UE pointer j at an initial stage of transmission. Therefore, at the initial stage of transmission, the UE proceeds to step 404 from step 402.
In step 404, the UE checks the buffer. If the buffer contains data to transmit, the UE proceeds to step 406. If the buffer contains no data to transmit, the UE proceeds to step 426 and ends the process. In step 406, the UE sets the buffer occupancy, the maximum transmission power, the Node B pointer 502, and TFCS. Although not shown in FIG. 4, the UE transmits at the initial stage of transmission the data stored in the buffer by means of the data rate corresponding to the TFC indicated by the received UE pointer j.
In step 408, the UE checks the buffer occupancy at a particular time interval in the course of the data transmission. According to the quantity of the data stored in the buffer, the UE may report the quantity to the Node B. The UE determines an optimum data rate in consideration of the maximum transmission power and the quantity of the data stored in the buffer. Further, the UE selects a TFC corresponding to the determined data rate from the TFCS. The selected TFC is set as TFCi. Referring to FIG. 5, i has one value from among 0 through 10.
In step 410, the UE compares the TFC corresponding to the data rate of current transmission with the TFC selected in step 408. In step 410, p refers to a level of the TFC corresponding to the data rate of the current transmission. Referring to FIG. 5, p has one value from among 2 through 10. The reason why p cannot have a value of 0 or 1 is that data cannot be transmitted at a higher data rate than the data rate assigned to the Node B. As a result of the comparison, the UE proceeds to step 412 if i is larger than p and proceeds to step 414 if i is not larger than p.
In step 412, the UE requests the Node B to assign a data rate that is one step higher and receives a response to the request. In consideration of the reception interference level, the Node B determines whether or not to perform assignment of the data rate requested by the UE. In step 416, the UE determines the response from the Node B. According to the result of the determination, the UE performs step 422 when information “DOWN” has been received and performs step 420 when information “KEEP” has been received. The UE performs step 418 when information “UP” has been received.
In step 414, the UE compares i and p. According to the result of the comparison, the UE performs step 420 when i and p are equal and performs step 422 when i and p are not equal.
In step 418, the UE replaces the current TFC by a new TFC one step higher than the current TFC. In step 418, the UE maintains the current TFC. In step 422, the UE replaces the current TFC by a new TFC one step lower than the current TFC. Table 1 shows an example of the result by the process from step 418 to step 422.
TABLE 1Current TFCStep 418Step 420Step 422TFC 6TFC 5TFC 6TFC 7
In step 424, the UE transmits the data stored in the buffer at the data rate corresponding to the TFC set in one of steps 418, 420, and 422. As shown in FIG. 4, the UE requests data rate assignment to the Node B only when the UE wants a higher data rate than a current data rate and does not request data rate assignment to the Node B when the UE wants a lower data rate than the current data rate.
FIG. 6 illustrates variance in reception interference of the Node B according to the process shown in FIG. 4. As shown in FIG. 4, the data rate variance is adjusted with reference to one level, so that the variance becomes less abrupt and fluctuating. Therefore, as opposed to FIG. 1, FIG. 6 shows ‘margin’ with a reduced width. The reduction of the ‘margin’ increases the target interference level target_2, thereby enabling efficient use of radio resources. ‘G’ in FIG. 6 represents the difference between target_1 and target_2.
In the conventional system as described above, a radio resource is assigned for signaling between the UE and the Node B, thereby causing capacity reduction of the system and causing time delay due to the signaling transmission. Further, because scheduling is necessary in order to assign radio resources to a plurality of UEs, the Node B has an increased complexity. Therefore, a solution for solving the above-described problems is highly required.